It is widely known in the art, particularly in the manufacture of polymeric films, to orient the film by stretching under heat. It is also known that certain properties of polymeric films in particular, such as tear strength, tensile strength, light transmission, haze, etc. can be improved by stretching the film. Examples of such polymeric film include polypropylene, polystyrene, nylon, polyimides, polyesters, polycarbonates and polyolefins. Such films will be referred to herein as "films." The orientation of polymeric films can be uniaxial, i.e., in either the longitudinal or lateral direction, or biaxial, i.e., in both directions at right angles to one another.
Generally, the biaxial orientation of films is a multistep process. First, a molten polymer is cast in a continuous molten sheet and quenched to cool the film below the glass transition temperature of the polymer to form a self-supporting amorphous sheet. The longitudinal edges of the sheet of film are typically formed to be thicker than the rest of the film or are provided with thickened beads of polymeric material. The film then undergoes stretching processes to orient and crystallize the film. A longitudinal stretcher heats the film above the glass transition temperature of the polymer and stretches the sheet of film 2 to 10 times in the longitudinal or machine direction and then cools the film. A tenter frame stretcher heats the film above the glass transition temperature and stretches the film 2 to 10 times in the lateral or widthwise direction. These stretching operations may occur in sequence or they may occur simultaneously.
Several optional steps may be employed in the above process, depending upon the particular desired use of the film. For example, subcoatings such as gelatin or resin may be applied to the as cast sheet of film prior to stretching. Likewise, special purpose coatings such as antistatic, matte, or acrylic coatings may be applied, typically after stretching. In addition, it may be desired to heat the film further in the tenter frame, prior to cooling it below the glass transition temperature, to heat-set and further crystallize the film. This step, as is known from Alles, U.S. Pat. No. 2,779,684, may be followed by a heat relaxing step to pre-shrink the film and stabilize its planar dimensions. It is also possible for the film to undergo surface treatments such as flame treatment or corona discharge.
A longitudinal stretcher utilizes two pairs of nip rollers positioned at the entrance and exit of the machine, with the rollers at the exit moving at a greater rate of speed to stretch the film in the machine direction. Because contact with the rollers may damage the film surface, the film does not engage either set of rollers unless it is below the glass transition temperature. Thus, in the stretcher the film must be heated above the glass transition temperature, stretched and then cooled below the glass transition temperature which necessarily requires a long span between the entrance and exit nip rollers. A long span in the stretcher, however, is known to cause a problem in that the lateral dimension of the film will tend to decrease as the film is being longitudinally stretched, a condition known as "neck-in."
In order to prevent neck-in from occurring, it is necessary to retain the longitudinal edges of the film from being pulled inward, and thereby maintain the lateral dimension of the film during stretching. Several methods of constraining the longitudinal edges of the film while stretching are known. U.S. Pat. No. 3,124,834 to Vandierendonck and U.S. Pat. No. 3,132,375 to Koppehele, for example, teach the use of rails positioned on either side of the stretcher. The rails are provided with channels which are adapted to receive therein the beaded or thickened longitudinal edges of the film. It is known to utilize a plurality of grippers positioned above and below the plane of the film and on both sides of the stretcher to constrain the longitudinal edges of the film. A predetermined, fixed clearance is maintained between the upper and lower grippers to provide a channel for the film, with the thickened edges of the film being disposed on the outside of the grippers. To insure proper operation, it was necessary to maintain the gripper clearance within narrow limits as compared to the average thickness of the film. If the clearance was too narrow, the grippers would cut a groove into the film, resulting in "dusting" of the polymeric material. The term "dusting" refers to the appearance on the film of dried flakes or particles (i.e., dust) of polymeric material. If the gripper clearance was too large, the thickened edges of the film would pull through the grippers, resulting in neck-in. A typical example would be for the gripper clearance to be no more than 0.002 inches [0.0051 cm] greater than the average thickness of the film. Because the thickness of the film changes as the film is stretched, however, the gripper clearance had to be set within the narrow tolerance limits based upon the average thickness of the film at the location of that particular gripper pair. A somewhat greater clearance tolerance was acceptable during the first phase of stretching, where the film is not thoroughly heated and is thus resistant to stretching.
The need to maintain the gripper clearance within the narrow tolerance limits during the stretching operation resulted in several disadvantages. It was common, for example, for the grippers to move out of adjustment, which necessitated periodic stoppage of the stretcher to check and readjust the gripper clearance. Similarly, if several types of films having different thicknesses were to be run on the same stretcher, it was necessary to shut down the stretcher and manually set the clearance for each gripper pair with a feeler gauge. Because of the large number of grippers typically employed in the stretcher, this tedious and labor intensive task would take upwards of eight hours to complete and would inevitably result in at least some of the grippers being missed completely or being improperly adjusted. A further disadvantage was that the gripper clearance, as a function of average film thickness, was often difficult to determine, particularly when manufacturing new or different films. As the line speeds in the film manufacturing process increase, these disadvantages become more acute and the costs in terms of manufacturing efficiency rise substantially.